We present Leon, a system for developing functional Scala programs annotated with contracts. Contracts in Leon can themselves refer to recursively defined functions. Leon aims to find counterexamples when functions do not meet the specifications, and proofs when they do. Moreover, it can optimize run-time checks by eliminating statically checked parts of contracts and doing memoization. For verification Leon uses an incremental function unfolding algorithm (which could be viewed as k-induction) and SMT solvers. For counterexample finding it uses these techniques and additionally specification-based test generation. Leon can also execute specifications (e.g. functions given only by postconditions), by invoking a constraint solver at run time. To make this process more efficient and predictable, Leon supports deductive synthesis of functions from specifications, both interactively and in an automated mode. Synthesis in Leon is currently based on a custom deductive synthesis framework incorporating, for example, syntax-driven rules, rules supporting synthesis procedures, and a form of counterexample-guided synthesis. We have also developed resource bound invariant inference for Leon and used it to check abstract worst-case execution time. We have also explored within Leon a compilation technique that transforms realvalued program specifications into finite-precision code while enforcing the desired end-to-end error bounds. Recent work enables Leon to perform program repair when the program does not meet the specification, using error localization, synthesis guided by the original expression, and counterexample-guided synthesis of expressions similar to a given one. Leon is open source and can also be tried from its web environment at leon.epfl.ch.